Coating compositions applied by electrodeposition are used extensively in various industries and particularly to coat or paint automobiles. In recent years the performance standards imposed by the automobile industry on paint formulations have become increasingly rigid particularly with regard to corrosion resistance and throwing power. At the same time, shorter cure time of the coatings are being demanded. Paint manufacturers have made efforts to meet these demands through careful selection of pain components such as the binding media, specific pigments, and the like. For example, it has been found that the corrosion resistance of a paint can be influenced, or modified, through the use of anti-corrosive pigments and/or binding media which are unsaponifiable to the greatest extent possible, and which have sufficient functionality for cross-linking reactions during the curing process.
The throwing or penetration of a paint composition, defined herein as the property of an electrodeposition paint to deposit on places which are shielded from the electrode whereby only a low current density is available to effect deposition, can be improved through modification of the binding agent either by the neutralizing agent employed or by the binder components. Moreover, throwing power can be increased by introducing auxiliary electrodes into the electrodeposition tank to reach the shielded zones. This later method is only accepted by the paint consumer as a last resort since it is costly, both with regard to personnel and equipment. It is also possible to improve the throwing power of electrodeposition paint media by increasing the conductivity of the paint. This can be done either by adding low molecular weight resol carboxylic acids which participate in the film formation, or by neutralizing an adequate binding agent with an inorganic base, particularly with KOH. However, if these methods are used, in many instances the corrosion resistance is substantially reduced. Optimization of both properties in the same formulation is largely impossible. An additional way of improving the throwing power is to develop binding media which have an extremely high electrical resistance of the deposited film. Deposition of such paint media requires very high voltages which in turn necessitates very costly equipment.